Impact Research, Geochemistry and Cosmochemistry, Planetary Geology
Laboratories in research group: Gamma spectroscopy, multiparameter coincidence spectrometry, stable isotope mass spectrometry, sample preparation, optical microscopy
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C. Koeberl is editor of the Geological Society of America Bulletin, from 2009 - 2015.
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Impact is a unique, short-time, high-energy geological process. The importance of impact cratering on terrestrial planets is obvious from the abundance of craters on their surfaces. On Earth, active geological processes rapidly obliterate the cratering record. To date only about 170 impact structures have been recognized on the Earth's surface. The come in various forms, shapes and sizes, from 300 km to less than 100 m in diameter, from Recent to 2 billion years in age. Mineralogical, petrographic, and geochemical criteria are used to identify the impact origin of such structures or related ejecta layers. An aspect of impact cratering that may be underestimated is the influence of impacts on the geological and biological evolution of our own planet. Even the impact of relatively small asteroids or comets can have disastrous consequences for our civilization. There is a 1 in 10,000 chance that a large asteroid or comet 2 km in diameter (corresponding to a crater of about 25-50 km in diameter) may collide with the Earth during the next century, severely disrupting the ecosphere and annihilating a large percentage of the Earth's population. Understanding of impact structures, their formation processes, and their consequences should be of interest not only to earth and planetary scientists, but also to society in general. The biological evolution of our planets is punctuated by mass extinction events, of which the one 65 million years ago, which marks the Cretaceous-Tertiary boundary, is probably the best known one. Abundant impact debris marks this boundary, providing a clear link with a major impact event. The Chicxulub impact structure in Mexico, about 200 km in diameter, which resulted from the impact of an about 10-km-diameter asteroidal body, has been identified as the culprit. Several other mass extinctions, most notably the Late Devonian, Permian-Triassic, Triassic-Jurassic, and Jurassic-Cretaceous ones, have been linked to possible impact events as well, although in these cases the evidence is not (yet) strong enough to allow unambiguous conclusions.
For a current list of terrestrial impact craters, see: http://www.unb.ca/passc/ImpactDatabase/
El’gygytgyn: a very special meteorite impact crater
The El’gygytgyn crater formed 3.6 million years ago by the impact of a large meteorite or asteroid, about 1 km in diameter. The crater and the lake that fills most of it are of scientific interest for two main reasons. First, this is the only known meteorite impact crater that formed in acid volcanic rocks and thus it offers the unique possibility to study the impact and shock effects on such rocks, which has implications for comparative planetology...
current research projects: Drilling at the Bosumtwi meteorite impact crater, Ghana
The 10.5-km-diameter 1.07 Ma Bosumtwi impact crater was the subject of an multidisciplinary and international drilling effort of the International Continental Scientific Drilling Program (ICDP) from July to October 2004. Sixteen different cores were drilled at six locations within the lake, to a maximum depth of 540 m. A total of about 2.2 km of core material was obtained. Desite of some technical and logistical challenges, the project has been very successful and the first scientific results became available in early 2006. As part of this international project, the Austrian contribution in studying the drill cores consists of geochemical, petrographic, mineralogic, and petrophysical investigations.
current research projects: Chesapeake Bay impact structure: ICDP/USGS deep drilling project
The Chesapeake Bay Impact Structure Deep Drilling Project (CBIS Project) completed its coring operations during September-December 2005 and April-May 2006. Cores were collected continuously to a total depth of 1,766 m. The recovered section consists of 1,322 m of impactites beneath 444 m of post-impact continental-shelf sediments.
The CBIS Project is a joint venture of the International Continental Scientific Drilling Program (ICDP) and the U.S. Geological Survey (USGS). Project activities began with a planning workshop in September 2003 that was funded by the ICDP, hosted by the USGS, and attended by 63 scientists from 10 countries. A resulting funding proposal to ICDP was accepted in late 2004, and additional drilling funds were authorized by the USGS. Field operations began with site preparation in July 2005, and coring began in September 2005. DOSECC, Inc., was the general contractor for the drilling operations throughout 2005. The NASA Science Mission Directorate, ICDP, and USGS provided important supplementary drilling funds in November-December 2005 that permitted coring of the deeper part of the impact structure. Studies of post-impact sediments were supported by the U.S. National Science Foundation (NSF), Earth Science Division, Continental Dynamics Program.
Buried at shallow to moderate depths beneath continental-margin sediments in southeastern Virginia, USA (Fig. 1), the late Eocene Chesapeake Bay impact structure is among the largest and best preserved of the known impact structures on Earth (Poag et al., 2004). It is the second largest among only a handful of known impact structures that formed in a marine setting (surpassed in size only by the Chicxulub structure in Mexico, the subject of an ICDP drilling project in 2001-2002). It is the source of one of only four tektite strewn fields that are currently known on Earth, the North American tektite strewn field (Koeberl et al., 1996). The Chesapeake Bay impact structure consists of a 38-km-wide, strongly and deeply deformed central zone surrounded by a shallower outer zone of sediment collapse known as the annular trough. Collectively, these two zones have a diameter of about 85 km and a distinctive shape that is generally referred to as an "inverted sombrero".